1. Field of the Invention
The present invention relates to a photoelectric conversion element.
2. Description of the Related Art
Conversion efficiency of a photoelectric conversion element is reduced due to a variety of causes, and various countermeasures have been devised. For example, a back-contact structure in which a collection electrode is not formed over a light-receiving surface and has absolutely no shadow loss, has been suggested (for example, see Non-Patent Document 1). However, in the case of a photoelectric conversion element that receives light that has passed through the atmosphere, since a refractive index of the photoelectric conversion element is higher compared to the atmosphere, a portion of incident light is reflected at the light-receiving surface of the photoelectric conversion element. As a result, light that enters the element is reduced, and there occurs a phenomenon in which apparent conversion efficiency of the photoelectric conversion element is reduced.
In view of this, a technique is known in which an asperity structure (a so-called textured structure) with a longer period (pitch) than the wavelength of incident light is provided over a light-receiving surface of a photoelectric conversion element, and by one structure receiving light that is reflected by another structure, reduction of conversion efficiency is prevented.
Also, a phenomenon in which a portion of light that has entered inside the photoelectric conversion element is not absorbed by a photoelectric conversion layer and goes out of the element is also one cause for reduction in the conversion efficiency of the photoelectric conversion element. Specifically, there is a case in which a portion of light that has entered from the light-receiving surface of the photoelectric conversion element is not absorbed by the photoelectric conversion layer and is reflected by another surface and released from the light-receiving surface. This phenomenon occurs more conspicuously as the photoelectric conversion layer of the photoelectric conversion element becomes thinner.
In view of this, a method is known for forming a structure that easily absorbs light, by providing over a light-receiving surface of a photoelectric conversion element a textured structure with about the same period as a wavelength of incident light, to reduce a component of the light that travels toward another surface from the light-receiving surface in the shortest distance, and increase a component that travels inside the photoelectric conversion element.
On the other hand, a phenomenon is known in which it is difficult for light to be reflected off a surface of an object provided with a fine structure with a period that is sufficiently shorter than an incident light. As an example of such a fine periodic structure, a fine structure called a moth-eye structure is known, and is used in a reflection prevention film. Furthermore, there have been attempts at improving conversion efficiency by providing such a fine periodic structure over a light-receiving surface of a photoelectric conversion element.